The strategy of utilizing microorganisms, chiefly bacteria contained in an activated sludge, to effect breakdown of organic wastes in influent streams, while simultaneously removing nutrients, is now almost universal in the field of sewage treatment. This raw sewage has a relatively high biological oxygen demand (BOD), and the breakdown products are typically lower molecular weight volatile fatty acids (VFA) such as acetic, propionic, or butyric acids. The composition is also high in suspended solids. Nitrogen is present as ammonia and organic, and phosphorous is present as inorganic phosphates.
It is known that the naturally occurring populations of microorganisms found in activated sludge are highly diverse, and represent a spectrum of genera ranging from strict aerobes to facultative anaerobes to obligate anaerobes. Each of these classes of organisms under appropriate manipulation can achieve some objective of the waste treatment process. Increasingly, it has become an objective of waste water treatment processes to remove nutrients such as total nitrogen including organic nitrogen, ammonia nitrogen, and oxidized nitrogen, and phosphates in addition to achieving removal of organic matter, which can affect delicate ecological balances. An understanding of the metabolism and catabolism of different classes of microbes has led to the design of various treatment protocols taking advantage of these natural processes.
Organic compounds provide food for bacterial growth. The organics, both simple and complex, contained in waste water fuel this growth. Under aerobic conditions, three types of metabolism can occur: (1) substrate oxidation in which organic compounds are converted to carbon dioxide and water; (2) synthesis in which organic compounds and nutrients are converted to cell protoplasm; and (3) endogenous respiration in which protoplasm is converted to carbon dioxide, nutrients, and water, as described in Metcalf & Eddy, Waste Water Engineering, 3rd ed., McGraw-Hill: 1991. In addition, energy and a metabolizable carbon source are also needed for nutrient utilization. Under anaerobic conditions, organic compounds can be further fermented to VFAs, primarily by the facultative species. The two principal nutrients requiring removal from waste water are inorganic phosphate and nitrogenous compounds. Influent waste water typically contains organic nitrogen and ammonia in the form of ammonium (NH.sub.4.sup.+). Hydrolysis of organic nitrogen and conversion of ammonia to free nitrogen gas (N.sub.2) which can readily be stripped from solution to the atmosphere requires two distinct processes. During nitrification, ammonia is converted first to nitrite (NO.sub.2.sup.-) by autotrophic oxidation involving Nitrosomonas spp. and related organisms, followed by further oxidation to nitrate (NO.sub.3.sup.-) involving Nitrobacter spp. A relatively broad range of heterotrophic facultative organisms then convert nitrate to free nitrogen (N.sub.2) in a series of steps. The basic multi-step process for nitrification and denitrification is set forth in the following reactions:
Nitrification: EQU NH.sub.4.sup.+ 1.5 O.sub.2 .fwdarw.NO.sub.2.sup.- +2H.sup.+ +H.sub.2 O(1)
(Nitrosomonas) EQU NO.sub.2.sup.- +0.5 O.sub.2 .fwdarw.NO.sub.3.sup.- (Nitrobacter)(2)
Denitrification: EQU NO.sub.3.sup.- +organic carbon.fwdarw.N.sub.2 +CO.sub.2 +OH.sup.- (facult.)(3)
Studies have shown that step (1) is rate limiting for nitrification and that Nitrobacter converts NO.sub.2.sup.- as an electron acceptor very quickly to NO.sub.3.sup.-. Meanwhile, denitrification is dependent on the availability of organic carbon sources.
It will be apparent that the nitrogen removal process requires first an aerobic step in which oxidation of ammonia to nitrate occurs (nitrification), followed by an anoxic step in which facultative organisms convert nitrate and nitrite to free nitrogen which can be released (denitrification). The earliest and most basic biological water treatment utilized constant aeration. These are of two treatment methods: fill, reactions and draw, and conventional flow through reaction followed by settling.
In more recent fill, reactions and draw, waste water is introduced to a single tank containing activated sludge. Alternating anaerobic/anoxic and aerobic phases are carried out to attain carbonaceous organic oxidation, nitrification, and denitrification. After settling, the clarified water is drawn off. In the multi-cell system, primary clarified water is mixed with activated sludge to form a mixed liquor, which is then passed through multiple aerobic/anoxic cells in a continuous flow process, and finally it enters a secondary clarifier. A portion of the sludge which settles out is returned to be mixed with waste water to form the mixed liquor. The aeration step helps to create biomass under the two aerobic processes outlined above, and also to nitrify ammonia. Denitrification then occurs to some extent upon establishment of anoxic conditions in the anoxic cells and secondary clarifier. In the latter, denitrification depends only on endogenous respiration.
Modern systems also seek to remove phosphorus species while simultaneously exchanging VFAs for phosphates. Removal of phosphates occurs in two steps and is mediated by a group of phosphorous rich microorganisms (Bio-P), principally Acinetobacter spp. and some Aeromonas. These organisms, when present in sludge passing through an anaerobic zone, use stored energy in the form of poly-phosphate to absorb food materials, principally VFA, and store it as poly-.beta.-hydroxybutyrate (PHB). In the process, the organisms release phosphates as the polyphosphates are broken down to release energy. This treatment zone must be anaerobic rather than anoxic, so that it is depleted of nitrates which would otherwise inhibit phosphate release and VFA absorption by the microorganism. Occasionally, raw waste water contains oxidized nitrogen species which may inhibit the process.
In the second step of phosphate removal, the aerobic bacteria contained in the sludge now moving through an aerobic zone metabolize the PHB and take up phosphates as biomass increases. Since more phosphate is taken up by the Bio-P organisms than was previously released, the difference is known as luxury uptake. In many conventional processes, VFAs from primary sludge fermentation is added to provide a carbon source for growth, and a low molecular weight carbonaceous compound such as acetic acid or methanol is added to provide an organic carbon source during denitrification. As cell growth depletes the absorbed organic carbon source with concomitant phosphorus uptake, the organisms switch to endogenous respiration with formation of flocks of senescent cells which settle out typically in a secondary clarifier.
The metabolic characteristics of these classes of organisms have been exploited in configuring a number of industrial processes designed to improve the efficiency of waste water treatment. In the basic A/O system (a single-sludge suspended growth system that combines anaerobic and aerobic sections in sequence), two successive tanks or basins are provided. Influent water first undergoes an anaerobic digestion step in which organics are fermented to VFAs along with phosphorous release and VFA absorption, followed by an aerobic step in a separate tank. The effluent is then further purified by settling in a clarifier From a nutrient standpoint, denitrification can occur in the first tank, with further nitrification of ammonia and stripping of nitrogen gas in the second tank. In this process, the recycling of sludge is important for two reasons: the biomass acts as a source of mixed liquor in the first tank, and the recycled nitrates are denitrified. Phosphates are released under the anaerobic conditions of the first tank, and taken up under the aerobic conditions in the second tank. Examples of a basic A/O type process are disclosed in U.S. Pat. Nos. 4,162,153 (Spector) and 4,522,722 (Nicholas).
Even though there is a coupling of anaerobic and aerobic processes, this system is relatively inefficient, with large volumes of fluid and long retention types. Inorganics, nutrients, and organic matter escape into the clarifier because not all of the dissolved material is distributed properly. Another source of inefficiency is the constant dilution of raw material in the anaerobic tank with recycled sludge containing oxidized nitrogen and new influent.
There are many modifications of the basic A/O type process, which can generally be divided into linear versus sequencing (nonlinear) categories. Variations of the A/O linear configuration include the A.sup.2 O process which includes separate anaerobic, anoxic, and aerobic zones with two recycle loops, one from the final clarifier to the anaerobic zone, and one from the aerobic outlet to the anoxic zone. The A.sup.2 O system splits the anaerobic and aerobic zones to several cells, and is very similar to the Bardenpho process. The advantage of this system is that it does not compromise the anaerobic zone by recycling material containing high levels of nitrates. Rather the high nitrate material is returned to anoxic conditions for denitrification. The five stage Bardenpho process adds a second anoxic and aerobic zone in series to the anaerobic, anoxic and aerobic A.sup.2 O system, but retains the A.sup.2 O recycle loops. While theoretically increasing the capacity of the system, it also has the advantage of combining the nutrient/BOD reducing recycle steps with a separate anoxic, aerobic cycle which treats the entire effluent volume.
Other linearly configured treatment systems are disclosed in U.S. Pat. No. 4,271,185 (Chen) in which a second oxic cell is provided after settling and prior to mixing to form mixed liquor, U.S. Pat. No. 4,488,967 which contains a number of linear treatment cells connected by bottom disposed apertures, and U.S. Pat. No. 4,650,585 (Hong) which has a series of anaerobic cells, and aerobic cells interconnected within a treatment series by bottom disposed apertures, but where the anaerobic cell series is connected to the aerobic series by a top disposed aperture, which in turn communicates through a top aperture with a clarifier. An interesting variation is disclosed in U.S. Pat. No. 5,160,043 (Kos) in which recycled sludge from the oxic tank is returned to the anaerobic tank after being retained in an exhaust tank to deplete nitrate levels. Another more complex linear-type system is disclosed in U.S. Pat. No. 5,213,681 (Kos) in which a series of anaerobic/aerobic treatment loops containing an exhaust tank are connected together in series with a terminal recycle after clarification to the influent line.
In the alternating or sequencing reactor systems, mixed liquor or treatment sludge can be directed to more than one tank destination at various times. Thus, a given tank can carry out one treatment process in one step and another treatment process in a different step. There is generally a more efficient use of equipment because each tank or treatment cell is not dedicated to a single treatment step. This provides for considerable flexibility in designing treatment protocols, especially in varying treatment times for different steps in response to the content of the influent.
An early sequencing system is disclosed in U.S. Pat. No. 3,977,965 (Tholander) in which influent is directed to one of two raceways interconnected by a valved conduit. Water entering one raceway can be treated under aerobic or anaerobic condition as desired, passed to the second raceway also capable of varied treatment, and is then discharged to a large clarifier. In a second cycle, influent is directed to the second raceway, passed to the first, and is discharged to the same clarifier. These systems are also known as DE-Ditch processes when influent and mixed liquor is first conditioned in an anaerobic tank. In a variation, a clarifier can be eliminated by using, alternatively, one or the other ditch as a settling container, with clarified water being discharged over an adjustable weir. An advantage of the process is creation of an anoxic zone in a non-aerated ditch, while providing a carbon source for denitrification, in this case by adding influent waste water containing degradable carbon.
Finally, U.S. Pat. No. 5,228,996 (Lansdell) discloses an alternating system having three series of cells linearly interconnected for continuous flow operation in which two of the three cell series are operated aerobically at any given time, and one series operates anoxically. At each treatment cycle, a different set of two series is aerobic, and the other set is quiescent for settling. The system operates without a separate clarifier, and is not equipped with a sludge return. This is possible because the activated sludge is alternately subjected to anoxic or aerobic conditions by changing the conditions in the respective cell series. The alternating conditions thus are the biological equivalent of a return cycle to the counterconditions of an earlier treatment phase.
In a variation of Tholander, U.S. Pat. No. 5,137,636 (Bundgaard) combines the alternating two tank anoxic/aerobic treatment strategy with a second aerobic treatment cell followed by a clarifier. Clarified sludge is returned to the inlet manifold. Phosphate removal is surprisingly efficient in this system which does not contain an ostensible anaerobic zone.